/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.flink.batch.tests.util;

import com.sun.nio.file.SensitivityWatchEventModifier;

import javax.annotation.concurrent.NotThreadSafe;

import java.io.Closeable;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardWatchEventKinds;
import java.nio.file.WatchEvent;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;

import static org.apache.flink.util.Preconditions.checkNotNull;

/**
 * A synchronization aid that allows a single thread to wait on the creation of a specified file.
 */
@NotThreadSafe
public class FileBasedOneShotLatch implements Closeable {

	private final Path latchFile;

	private final WatchService watchService;

	private boolean released;

	public FileBasedOneShotLatch(final Path latchFile) {
		this.latchFile = checkNotNull(latchFile);

		final Path parentDir = checkNotNull(latchFile.getParent(), "latchFile must have a parent");
		this.watchService = initWatchService(parentDir);
	}

	private static WatchService initWatchService(final Path parentDir) {
		final WatchService watchService = createWatchService(parentDir);
		watchForLatchFile(watchService, parentDir);
		return watchService;
	}

	private static WatchService createWatchService(final Path parentDir) {
		try {
			return parentDir.getFileSystem().newWatchService();
		} catch (IOException e) {
			throw new RuntimeException(e);
		}
	}

	private static void watchForLatchFile(final WatchService watchService, final Path parentDir) {
		try {
			parentDir.register(
				watchService,
				new WatchEvent.Kind[]{StandardWatchEventKinds.ENTRY_CREATE},
				SensitivityWatchEventModifier.HIGH);
		} catch (IOException e) {
			throw new RuntimeException(e);
		}
	}

	/**
	 * Waits until the latch file is created.
	 *
	 * <p>When this method returns, subsequent invocations will not block even after the latch file
	 * is deleted. Note that this method may not return if the latch file is deleted before this
	 * method returns.
	 *
	 * @throws InterruptedException if interrupted while waiting
	 */
	public void await() throws InterruptedException {
		if (isReleasedOrReleasable()) {
			return;
		}

		awaitLatchFile(watchService);
	}

	private void awaitLatchFile(final WatchService watchService) throws InterruptedException {
		while (true) {
			WatchKey watchKey = watchService.take();
			if (isReleasedOrReleasable()) {
				break;
			}
			watchKey.reset();
		}
	}

	private boolean isReleasedOrReleasable() {
		if (released) {
			return true;
		}

		if (Files.exists(latchFile)) {
			releaseLatch();
			return true;
		}

		return false;
	}

	private void releaseLatch() {
		released = true;
	}

	@Override
	public void close() throws IOException {
		watchService.close();
	}
}
